The worker server was heavily tied to OSBuildJob(Result). Untie it so
that it can deal with different job types in the future.
This necessitates a change in the jobqueue: Dequeue() now returns the
job type, as well as job arguments as json.RawMessage. This is so that
the server can wait on multiple job types with different argument
types.
The weldr, composer, and koji APIs continue to use only "osbuild" jobs.
The enum is redundant information that can be deduced from the job's
times: queuedAt, startedAt, and finishedAt. Not having it reduces the
potential for inconsistent state.
Now that the "old" `jobqueue` package was renamed to `worker`, add a new
package that contains an interface to an actual job queue. Also add two
implementations: fsjobqueue, a job queue backed by the file system, and
testjobqueue, which can be used as a mock implementation for testing.
These packages are not yet used.
